So how did Genghis Khan, one of history's cruelest conquerors, earn such a glowing environmental report card? The reality may be a bit difficult for today's environmentalists to stomach, but Khan did it the same way he built his empire — with a high body count.

Over the course of the century and a half run of the Mongol Empire, about 22 percent of the world's total land area had been conquered and an estimated 40 million people were slaughtered by the horse-driven, bow-wielding hordes. Depopulation over such a large swathe of land meant that countless numbers of cultivated fields eventually returned to forests.

In other words, one effect of Genghis Khan's unrelenting invasion was widespread reforestation, and the re-growth of those forests meant that more carbon could be absorbed from the atmosphere.

Weirdly, the author equates cooling the Earth with "a glowing environmental report card?" How did cold become green?

In fact, the world did substantially cool in the 14th century. The previous 300 warm years had brought prosperity and growth to Western Europe, in fact the first population growth in Europe since as early as 300AD. The commercial and intellectual regression that is often called the Dark Ages or the early Middle Ages (say 700-1000AD) is often attributed to a demographic collapse in Western Europe. There are many who credit, at least in part, this collapse for the fall of the Western Roman Empire.

The years 1000-1300 saw a real recovery, the first population growth for hundreds of years, and a number of important (though to us prosaic) technological, intellectual and societal advances. There are several factors behind this boom, but a large one is the Medieval Warm Period, where we can find records of certain crops (e.g. grapes in England) being grown far north of where they can be even today.

The early 1300's coincided with the return of cold, wet weather to Europe. Whether this is in part attributable to Genghis Khan's killing rampage, I can't say. But the effects were clear. The 1320's and 1330's saw a series of terrible harvests and resulting famines. By the 1340's, much of Europe was hungry and malnurished, weakening the population for the arrival of some rats carrying Bubonic Plague. Again, not a few historians have noted that the climate-change-induced famines of the early 1300's likely made the early plagues more virulent.

This world of failed harvests, starving, and plagues -- this is a greener world we should aspire to?